An electrochemical molten carbonate fuel cell stack consists of a plurality of assemblies comprising an anode electrode, an electrolyte filled matrix, a cathode electrode, and an interconnect plate or bipolar separator plate. The assemblies are arranged in series relationship to form the fuel cell stack.
A eutectic mixture of lithium carbonate and potassium carbonate in a ratio of 62 mole %/38 mole % is commonly used in molten carbonate fuel cells (MCFC) as an electrolyte. The eutectic mixture of lithium carbonate and potassium carbonate is commonly processed as a finely divided powder that is a solid at room temperature. The electrolyte may be pre-melted and re-processed to powder form. The two components of the electrolyte can also be purchased separately in bulk powder form and used as a simple physical mixture such as for example lithium potassium from Chemetall Foote Corporation product number 51039 and potassium carbonate from Harcros Chemicals, Incorporated, and no part number.
Methods of installing the electrolyte within the fuel cell at the point of assembly include the storage of said electrolyte within the electrodes or the electrolyte matrix of the fuel cell. These methods require that the electrolyte be evenly deposited upon the surface of said electrode or electrolyte matrix and the placement of the electrode/electrolyte assembly within an atmospherically controlled furnace and heating the assembly above the melting temperature of the electrolyte. An atmosphere within the furnace must be maintained with a partial pressure of carbon dioxide above that point that would result in decomposition of the carbonate electrolyte. Upon melting, the electrolyte will wick into the pores of said electrode or electrolyte matrix and be retained. Upon cooling, the electrolyte will solidify within the pores of said electrode or electrolyte matrix forming a highly fragile, glass-like sheet that is difficult to handle and process into the fuel cell assembly. Non-uniformity of distribution of the electrolyte within the electrode or electrolyte matrix results in variation of the mechanical properties of the electrode or electrolyte matrix such as compressive strength and permeability.
Another method of installing the electrolyte within the fuel cell at the point of assembly is the storage of said electrolyte within the flow channels of the bipolar separator plates of each cell of the fuel cell stack. Upon initial start-up of the fuel cell, the electrolyte will melt and wick into the porous electrodes and electrolyte matrix. Sufficient quantity of electrolyte is necessary to adequately fill the pores of the electrolyte matrix so as to create an impermeable seal that prevents mixing of reactant gasses. Additional electrolyte is required to wet the catalyst comprising the electrodes.
Another method of installing the electrolyte within the fuel cell at the point of assembly involves a combination of filled electrodes and filled flow channels.
Tape casting is a common method for producing MCFC electrodes. Tape cast electrodes are produced with polymer binders that result in very handle-able electrodes that are easily processed during the manufacturing process. It is desirable to avoid the added manufacturing processes associated with storage of electrolyte within the electrodes of the fuel cell during assembly.
Dry packing of electrolyte within the flow channels of the bipolar plate of the fuel cell presents manufacturing challenges that are not easily overcome. These challenges include storage of sufficient quantity of electrolyte to properly fill the pores of the electrolyte matrix to create the seal that prevents mixing of fuel cell reactants.
An alternative method of achieving sufficient packing density of the electrolyte in the flow channels is to prepare a slurry of powdered electrolyte and a solvent. Wet slurries achieve higher packing density than dry slurries. The slurry solvent may be any number of solvents or fluids that do not irreversibly alter the chemical composition of the electrolyte. Various solvents provide varying degrees of packing density of the electrolyte. U.S. Pat. No. 5,468,573 to Bregoli, et al teaches a method of preparing electrolyte slurry with a solvent comprising glycerin. The “573” patent states that aqueous based slurry is unsuitable due to water reacting with the electrolyte or other components.